The coating by means of thermal evaporation has been generally known for the past decades. The simplicity of the thermal evaporation is an essential reason for its extensive propagation in the field of coating methods, particularly in the manufacture of organic opto-electronic systems such as, for example, displays on the basis of organic light-emitting diodes (OLED).
During the thermal evaporation the material to be evaporated is heated in an evaporator in such a way that it changes over into the gaseous condition (evaporation or sublimation). After this, the material particles (molecules, among others) migrate to the envisaged deposition surface, as a result of their obtained kinetic energy or by means of diffusion processes (e.g. in a carrier gas) where they then condense. In a typical manner, the evaporation takes place under vacuum conditions. However, evaporations under reduced pressure or even at atmospheric pressure by means of inert carrier gas are also possible.
From practice, numerous techniques for the transfer of the material into the gas phase are known. In an uncomplicated form, the material to be evaporated is put into a crucible which is surrounded by a heating element (a filament). When the crucible is heated, the material is heated up either by heat radiation and/or heat conductivity and, in this way, the energy as required up to the transition into the gas phase is applied.
Evaporators for the coating technique are frequently designed in such a way that as much material as possible is deposited within the shortest possible time but, however, under stable conditions. In this case, the material should only be thermally stressed to a minor degree with the setting of an evaporator temperature as low as possible. This requirement is relevant particularly for organic materials because these tend towards transformation or destruction with an excessively high energy input. Then again, the stability of the material must also be observed with inorganic compounds.
In order to comply with the above-mentioned requirements, it was proposed to have a homogenous energy input into the material to be evaporated. In EP 1 132 493 A2 it is proposed for the evaporation of organic molecules for the coating of substrates to mix the evaporating material in a crucible with particles of metal or ceramic material which have a high level of thermal conductivity. Despite the improved thermal conductivity reaction with the addition of the metal or ceramic particles, there is the disadvantage that the energy input into the crucible is effected only indirectly. The energy transport through the particles is only inadequate as it is reduced at the contact points between the particles by the material to be evaporated. There can be a further disadvantage in the fact that, as a result of the loose arrangement in the crucible, the radiating characteristics of the evaporator change with a varied filling level. This has negative influences on the coating homogeneity and the process control. Moreover, the form of the crucible and, subsequently, also the form of the molecular beam is restricted.
From EP 0 561 016 A1 it is known to manufacture organic layers from waxes or urethanes for example by first impregnating a porous carrier with a melt or with a solution of an organic material and then heating up the carrier in a thermal evaporator in order to evaporate the organic material. As the accommodating capacity of the carrier is known, this technique enables a quantitative evaporation of predetermined material quantities. The use of the carriers, however, has a series of disadvantages, so that this conventional technique only has limited suitability for the fulfillment of the above-mentioned requirements. There is a first disadvantage in the indirect heating of the impregnated carriers in an evaporator crucible. The heat input from an external source is inhomogeneous so that there is subsequently a non-uniform evaporation and, therefore, a non-uniform coating. In order to counteract this problem, with the conventional technique porous particles or thin platelets are used as carriers. The particles have the above-mentioned disadvantage of a worsened heat transition. The platelet shape represents an undesirable restriction of the evaporation-capable material volume and/or the evaporator geometry. There is a further disadvantage in the fact that, with the conventional carriers which are designed for a loading with an impregnation, a complete elimination of any possibly existing solvents cannot be ascertained. With numerous applications, the presence of solvents during the evaporation process is however problematic because the solvents as contaminating substances disturb the layer formation and the layer properties. During the manufacture of, in particular, organic, opto-electronic structural elements, this can lead to a reduction of the efficiency or the life service duration.